With rapid economic development, mankind is facing critical problems, including the depletion of petrochemical resources and pollution caused by non-renewable energy resources such as gasoline and coal. It is imperative to find other sources of energy that have the advantages of high energy density, environmental friendliness, and sustainable development.
In order to solve this problem, many attempts to use a sodium resource, which is sufficiently present on earth, as a material in a secondary battery have been conducted. The sodium secondary battery shows a higher charge capacity and a higher discharge capacity, and has a longer cycle life.
According to the polarity of the solvent used in the electrolyte, the sodium secondary battery can be classified into two groups, i.e., aqueous electrolyte sodium secondary battery and non-aqueous electrolyte sodium secondary battery. Although the electrolyte sodium secondary battery has the advantages of having a low cost and being safe, but the aqueous electrolyte sodium secondary battery has a low applicability due to the low energy density.
In comparison with the conventional ionic liquid, the deep eutectic solvent has advantages of easier preparation, availability of raw materials, and high biocompatibility. Recently, research into electrochemical energy storage systems having a deep eutectic solvent have been increasing. It is important to develop the new electrolyte with a low cost, high cycle stability, wide operating potential, and ion conductivity similar to a conventional non-aqueous electrolyte for sodium secondary battery. Accordingly, there is a need to develop an electrolyte used in the sodium secondary battery for increasing the charging/discharging efficiency and capacity of the sodium secondary battery.